1. Field of the Invention
The present invention generally relates to guided transmission of optical signals. In particular, the present invention relates to a method of transmitting an optical signal in an optical transmission system and an optical transmission system which implements such a method. The present invention further relates to a method for selecting a transmission wavelength for transmitting an optical signal in an optical transmission system.
2. Description of the Prior Art
In an optical transmission system, information is transmitted by means of optical signals. Typically, an optical transmission system comprises at least one transmitter, the at least one transmitter being adapted to transmit a channel, i.e. an optical signal at a given wavelength. Optical signals are then transmitted through a link.
The link generally comprises one or more spans of optical fiber. During propagation along each span, a fraction of the optical signal power is absorbed by the optical fiber. Thus, at the end of each span, a respective amplifier may be provided in order to compensate the optical fiber absorption.
An optical transmission system may be either a single-channel transmission system or a multi-channel transmission system (e.g. Wavelength Division Multiplexing or, briefly, WDM transmission systems). The channel wavelengths are generally established by standards defining, among other parameters, the channel position, the channel wavelength tolerance and, in case of multi-channel transmission systems, the channel spacing. For instance, the Dense WDM standard defines an equispaced channel grid between 1530 nm and 1625 nm; the channel spacing is 0.8 nm and the channel wavelength tolerance is +/− about 0.1 nm. Another example is the Coarse WDM standard, which defines an equispaced channel grid between 1270 nm and 1610 nm; the channel spacing is 20 nm, and the channel wavelength tolerance is +/−6.5 nm.
Generally speaking, reducing the channel spacing results in a more expensive WDM transmission system. Indeed, closer channels require more costly transmitters, as the wavelength of the transmitter laser source must be more stable. This requires more accurate, and therefore more costly, temperature and feeding current control equipments. Further, reducing the channel spacing also increases the cost of multiplexers and demultiplexers, since they become more complex to design and to manufacture.
As already mentioned, in an optical transmission system information is transmitted by means of optical signals. More particularly, data to be transmitted comprise a bit sequence. Thus, the optical signal transporting such a bit sequence consists of a carrier at the channel wavelength which is modulated by the bit sequence. The width of the time frame required for transmitting a single bit (or bit period) determines the bit-rate of the optical transmission system, which is measured in bit/s. For instance, if the bit period is 100 ps, the bit-rate is 10 109 bit/s=10 Gbit/s.
Different modulation formats are known in the field of optical transmissions. In the following description, it is assumed that the bit sequence modulates the carrier though a binary amplitude modulation, wherein an optical pulse with amplitude substantially different from zero is associated to a first logical level “1”, while an optical pulse with amplitude substantially equal to zero is associated to a second logical level “0”. However, any other modulation format (frequency modulation, phase modulation, multi-level modulations . . . ) can be provided as well.
Typically, the width of each pulse is lower than or equal to the bit period, so that consecutive pulses do not temporally overlap. This allows the receiver to distinguish consecutive pulses and to associate them to corresponding logical levels “1” or “0”.
However, during propagation, pulses may change their shape, in particular their amplitude and their width, due to an effect which is termed dispersion.
The dispersion is an effect wherein waves at different wavelengths propagate in a material at different speeds. As a pulse comprises the superimposition of waves at different wavelengths, such waves propagate into an optical fiber at different speeds, thus modifying the pulse shape. In particular, the pulse width increases, and the pulse amplitude is correspondingly reduced. The pulse width increases by increasing the optical fiber length. Further, the pulse width increases according to a coefficient which is called chromatic dispersion, whose value depends on the optical fiber characteristics, and which is expressed in ps/nm/km. The chromatic dispersion of an optical fiber substantially is the delay (expressed in ps) of the propagation time of two waves having 1-nm spaced wavelengths, over a 1-km long span of said optical fiber.
It has to be noticed that chromatic dispersion is positive in case waves with higher wavelengths propagate at lower speed with respect to waves with lower wavelengths. On the contrary, the chromatic dispersion is negative in case waves with higher wavelengths propagate at higher speed with respect to waves with lower wavelengths.
Moreover, the chromatic dispersion of an optical fiber depends on the wavelength of the optical signal propagating along the optical fiber. Typically, in optical fibers for optical transmission systems, the chromatic dispersion increases with the wavelength, at least in the wavelength range which is typically used for optical transmission systems. The chromatic dispersion of such optical fibers becomes zero at a zero dispersion wavelength. For instance, optical fibers G.652, which are standardized by the homonymous ITU-T Recommendation, have zero dispersion wavelength at about 1310 nm. Further, optical fibers G.653, which are standardized by the homonymous ITU-T Recommendation, have zero dispersion wavelength at about 1550 nm. The zero dispersion wavelength of an optical fiber is generally defined with a zero dispersion wavelength tolerance, as it will be discussed in further details herein after. Such a zero dispersion wavelength tolerance may be indicated either into the relevant ITU-T Recommendation of the optical fiber, or into fiber specifications provided by optical fiber suppliers.
Generally speaking, dispersion affects the performances of an optical transmission system. Indeed, in an optical transmission systems, pulses are received by a receiver. The receiver associates to each pulse a respective logic value “1” or “0”. However, if the pulse shape is distorted due to dispersion (higher pulse width and lower pulse amplitude), the receiver is no longer able to properly associate each pulse to the respective logic value. It can be shown, by means of equations which are not reported into the present description, that, in case fiber non-linearity is negligible, the performance of an optical transmission system deteriorates by increasing the bit-rate and by increasing the accumulated dispersion, wherein the accumulated dispersion is defined as the product of the chromatic dispersion of an optical fiber and the length of said optical fiber. It has to be noticed that the performance of an optical transmission system also depends on other system parameters (modulation format, receiver threshold, and the like). However, the effects of this parameters will not be taken into account into the present description.
Thus, once the bit-rate and the other system parameters (modulation format, receiver threshold, and the like) of an optical transmission system have been set, a tolerated accumulated dispersion range can be estimated wherein system performances are acceptable. For instance, an optical transmission system with a bit-rate of 10 Gbit/s may have a tolerated accumulated dispersion range between −960 ps/nm and 960 ps/nm. If, in the same optical transmission system, the bit-rate is increased to 40 Gbit/s, the tolerated accumulated dispersion range becomes between −60 ps/nm and 60 ps/nm.
Thus, at the end of the optical fiber link of an optical transmission system, the accumulated dispersion of an optical signal must be comprised within said tolerated accumulated dispersion range.
A know technique for providing, in an optical transmission system, an optical signal with accumulated dispersion comprised within a tolerated accumulated dispersion range is the so-called dispersion compensation technique. Dispersion compensation technique consists in providing each span of a link with a respective dispersion compensator, which introduces on the optical signal a compensation dispersion, i.e. a dispersion with opposite sign with respect to the dispersion of the span. Such dispersion compensators may be implemented through Dispersion Compensating Fibers (DCF) or other dispersive components, such as Bragg gratings. Therefore, the accumulated dispersion of an optical signal at the end of the link is the sum of the accumulated dispersion of the spans and of the accumulated dispersion of the dispersion compensators. The compensation dispersions are tailored so that the accumulated dispersion at the end of the link is comprised within the tolerated accumulated dispersion range.
Such a dispersion compensation technique has some disadvantages. First of all, providing a dispersion compensator for each span increases the optical transmission system cost. Moreover, the dispersion compensators absorb a part of the optical signal power. Such absorption must be compensated, for instance, by increasing the optical power of the transmitter; this however increases both the cost of the transmitter and the complexity of the transmitter safety equipments. As an alternative, the absorption can be compensated by increasing the amplifier gain; this however increases both the amplifier cost and the complexity of the amplifier safety equipments. As an alternative, a double stage amplifier can be provided at the end of each span, and the respective dispersion compensator may be inserted between the first stage and the second stage of the amplifier; this however increases the cost of the amplifier. As an alternative, a higher sensitivity receiver may be provided; this however increases the receiver cost.